MEMS Device for Large Angle Beamsteering

ABSTRACT

An actuator element of a MEMS device is provided, which is fabricated using surface micromachining on a substrate. An insulating layer having a first portion contacts the substrate while a second portion is separated from the substrate by a gap. A metallic layer contacts the insulating layer having a first portion contacting the first portion of the insulating layer and a second portion contacting the second portion of the insulating layer. The second portion of the metallic layer is prestressed. Alternately, the actuator element includes a first insulating layer separated from the substrate by a gap. A metallic layer has a first portion contacting the substrate and a second portion contacting the insulating layer. A second insulating layer contacts a portion of the second portion of the metallic layer opposite the first insulating layer, where the second insulating layer is prestressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/052,018, entitled “MEMS Device for Large Angle Beamsteering,” filedAug. 1, 2018, which claims the benefit of and priority to U.S.Provisional Application Ser. No. 62/540,177, entitled “Post-ProcessingTechniques on MEMS Foundry Fabricated Devices for Large AngleBeamsteering,” filed on Aug. 2, 2017, and U.S. Provisional ApplicationSer. No. 62/587,734, entitled “Segmented Control of ElectrostaticallyActuated Bi-Morph Beams,” filed on Nov. 17, 2017, and U.S. ProvisionalApplication Ser. No. 62/589,610, entitled “Using Surface Micromaching toCreate Large Tip, Tilt, and Piston MEMS Beamsteering Structures,” filedon Nov. 22, 2017, and U.S. Provisional Application Ser. No. 62/667,647,entitled “Torsional Structures to Enable Large Angle Deflections,” filedon May 7, 2018, and U.S. Provisional Application Ser. No. 62/702,595,entitled “Torsional Springs to Enable Large Angle Tip/Tilt Beamsteeringusing MEMS,” filed on Jul. 24, 2018, the entireties of which areincorporated by reference herein.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to MEMS devices and, moreparticularly, MEMS devices capable of large angle deflections.

Description of the Related Art

Within the past decade, numerous researches have invested time in thedevelopment of micro-electro-mechanical systems (MEMS) micromirrorstructures which have the ability to deflect at large angles (greaterthan 20 degrees). These large tip/tilt micromirrors are ideal for manyapplications to include microscopy, biomedical endoscopy, lasercommunication, wavelength selectivity, optical tuning, scene generationand various other medical instrumentations. Although many of theseresearch efforts exhibit large tip/tilt angles, they generally do notinclude a piston motion for optical correction requirements or exhibithigh fill-factors for large area optical scanning applications. Therecurrently are no MEMS large angle beamsteering approaches which exhibitlarge tip/tilt and piston motion while exhibiting a fill-factor greaterthan 90%, which may be fabricated using surface micromachining. Currentstate of the art electrostatic designs have a maximum tip or tilt angleof ±28 degrees for a single element but generally do not possess bothcapabilities. Electro thermal designs have a maximum tip or tilt angleof ±40 degrees for a single element but also generally do not possessboth capabilities. No approach with a tip or tilt angle of greater than10 degrees are available which has a high fill-factor. Most if not alldesigns with a high fill-factor have tilt angles of less than 5 degrees.

Accordingly, there is a need in the art for MEMS micromirrors for largeangle beamsteering for numerous broadband steering and imagingapplications.

SUMMARY OF THE INVENTION

Micro-Electro-Mechanical Systems (MEMS) micromirrors have been employedin a wide range of optical applications for about two decades. However,scanning micromirrors are far less numerous, generally exhibit lowscanning angles (less than 20°) and typically in only one direction.Embodiments of the invention provide large angle, out-of-plane bimorphMEMS micromirrors fabricated in foundry processes as well as in-house.Through modeling and simulation, several techniques are possible to meetthe required large out-of-plane deflections needed for large anglebeamsteering. Both a serpentine and center anchored multi-beam approachhave been designed, modeled, fabricated, and tested to observedeflection and overall functionality of the structures. These structuresexhibit high, out-of-plane deformations as either a MEMS electrostaticand electrothermal actuators, which can then be integrated with an SOIor some other fabricated micromirror array to enable broadband steeringand imaging applications. The arrays are able to exhibit tip, tilt, andpiston motion due to the individual actuation design schemes which areutilized in each micromirror structure while maintaining a highfill-factor and is scalable to large aperture and array sizes. Thedesign methodology capitalizes on the inherent residual stresses inbimorph structures which possess different coefficients of thermalexpansions (CTE). Through precise material selection, and design control(i.e. structure length, material thickness, material CTE, depositiontemperature, and material layer composition), this inherent residualstress will be used to create the upward deflections required for thesesurface micromachined structures to enable large angle micromirrormovements.

Embodiments of the invention provide a MEMS device on a substrate, whichincludes a platform to which a micromirror may be attached or may beused as the fabrication point for the micromirror. A least one actuatorelement chain composed of a plurality of actuator elements is connectedto the platform. Actuation of the plurality of the actuator elements inthe actuator element chain causes motion in the platform, which can becontrolled to steer the mirror. The actuator elements of the pluralityof actuator elements form a chain, which may be arranged in a serpentineconfiguration in some embodiments or with a center contact configurationin other embodiments.

In some embodiments, the actuator elements of the MEMS device include aninsulating layer having a first contacting portion and a second portionseparated from an electrode by a gap. A metallic layer contacts theinsulating layer and has a first portion contacting the first contactingportion of the insulating layer and a second portion contacting thesecond portion of the insulating layer, the second portion of themetallic layer is prestressed. In some of these embodiments the secondportion of the metallic layer of the actuator element is tensilelyprestressed.

In other embodiments, the actuator elements of the MEMS device include afirst insulating layer positioned above the substrate and separated fromthe substrate by a gap. A metallic layer has a first contacting portionand a second portion contacting the insulating layer. A secondinsulating layer contacts a portion of the second portion of themetallic layer opposite the first insulating layer, where the secondinsulating layer is prestressed. In some of these embodiments, thesecond insulating layer of the actuator element is compressivelyprestressed.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description given below, serve to explain the invention.

FIG. 1A is MEMS structural device used for large angle beamsteeringillustrating an exemplary embodiment with a center contact design;

FIG. 1B is MEMS structural device used for large angle beamsteeringillustrating an exemplary embodiment with a serpentine based design withthe contact at the end of the first, longest cantilever beam, which is amodification of the serpentine design in FIG. 2 below;

FIG. 2 illustrates and exemplary serpentine design shown;

FIG. 3 illustrates an exemplary large angle beamsteering micromirrordesign concept using the center contact design approach;

FIG. 4A illustrates the deflection of the serpentine design in FIG. 2showing an upward deflection of about 60 μm;

FIG. 4B illustrates the deflection of the modified serpentine design inFIG. 1B with an upward deflection of about 145 μm;

FIG. 5A illustrates a peak, upward beam deflection of 150 μm foridentical center contact designs with the same physical actuatordimensions as shown in FIG. 1A with an aluminum metal layer ;

FIG. 5B illustrates a peak, upward beam deflection of 80 μm foridentical center contact designs with the same physical actuatordimensions as shown in FIG. 1A with a gold metal layer;

FIG. 6 illustrates the total upward deflection following release of themicromirror of FIG. 3;

FIG. 7A illustrates an overall design concept of the actuation assemblyfor large out-of-plane deflections as deposited layers prior to release;

FIG. 7B illustrates a post released structure of FIG. 7A showing theout-of-plane upward deflection;

FIG. 7C illustrates an SEM image of bimorph cantilever beams for anactual structure in a released configuration;

FIG. 8A illustrates PolyMUMPs foundry fabrication layers for anexemplary device;

FIG. 8B contains a table with material layer descriptions andthicknesses of the exemplary device in FIG. 8A;

FIGS. 9A-9C illustrate a fabrication sequence of the post-processedactuation assembly from baseline design received from a foundry to areleased structure;

FIG. 10 shows a deformed model of an electrothermal design as fabricatedin PolyMUMPs;

FIG. 11 shows a deformed model of the electrothemal design of FIG. 10with a post-processed SiN layer;

FIG. 12 shows a deformed model of the post-processed electrothermaldesign of FIG. 11 with a 300 C gold metal evaporation layer replacingthe PolyMUMPs gold layer;

FIG. 13A shows a deformed model of an electrostatic serpentine design;

FIG. 13B shows scanning electron microscope images of the electrostaticdevice in FIG. 13A;

FIG. 14A illustrates a torsional spring attachment which exhibits amoderately high piston and tip/tilt spring constants, which will maketilt of the micromirror platform difficult;

FIG. 14B illustrates the highest spring constants which make for a veryridged and stiff structure that nearly prohibits tilting events;

FIG. 14C illustrates a lower spring constant which enables tilting butis not ridged enough such that it creates some of the negative downwardmotion of the opposite actuator assembly;

FIG. 14D illustrates a lower spring constant for a tip/tilt event and isrigid enough for piston actuation;

FIG. 15A includes a SEM image of a completed, unreleased PolyMUMPs™micromirror actuation assembly;

FIG. 15B includes a SEM image of the same device in FIG. 15A followingsacrificial oxide release;

FIG. 15C includes a SEM image of a portion of FIG. 15B where thetorsional spring is fully extended under an actuation condition;

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the sequence of operations as disclosedherein, including, for example, specific dimensions, orientations,locations, and shapes of various illustrated components, will bedetermined in part by the particular intended application and useenvironment. Certain features of the illustrated embodiments have beenenlarged or distorted relative to others to facilitate visualization andclear understanding. In particular, thin features may be thickened, forexample, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention address the need in the art by enabling newand improved beamsteering systems with large beamsteering angles andhigh scanning speeds while exhibiting high fill-factor (greater than90%) arrays, which may be scalable to large aperture sizes as well asenable a multi-beam scanning capability at a low bias voltage.Embodiments of the invention have the potential to replace conventionalgimbal systems on platforms since these devices are nearly conformal,and eliminate all macro-scale moving mechanical parts of thecontemporary scanning/detector systems. Embodiments of the inventionwould be applicable to EO/IR beamsteering systems, imaging and scenegeneration systems, laser communications, multi-target search and track,among others. Conventional methods use gimbal systems which are slow,single beam beamsteering with no multi-target detect/track capability.Advantages of the embodiments of the invention include elimination ofmost mechanical/gimbal systems on a platform, can enable multi-beamsteering, low voltage, nearly conformal, adaptable and scalable to meeta wide range of applications with regards to steering angles andscanning speeds. These embodiments use MEMS to enable the large anglebeamsteering, scalable with regard to array size, flexible with regardsto material selection, wavelengths of interest, and deflection/steeringangles while being ideal for wideband applications.

FIGS. 1A and 1B illustrate two different structural concepts which weredesigned to capitalize on the inherent residual stress in materials inaddition to capitalizing on the different coefficient of thermalexpansion (CTE) of the bimorph material layers selected. FIG. 1A shows abasic center contact design concept with a contact pad removed formodeling purposes. This device may be biased by individually applying avoltage potential to each of the four actuators separately to create atip & tilt scenario, or all four actuators being biased simultaneouslyto create a piston motion for a micromirror to enable optical phasecorrection. FIG. 1B illustrates a modified serpentine design concept inwhich the actuators can also be biased individually or allsimultaneously to create the tip/tilt and piston motion as describedabove. These exemplary embodiments were determined for an exemplaryapplication where a physical beamsteering actuator was limited to 1 mm²or less. This choice was made to try and maximize the initial upwarddeflection while also maintaining a reasonable number of control linesfor individual actuation control as well as providing the necessarysensing lines to determine the precise location of the mirror withrespect to elevation and angle during device operation. From either ofthe embodiments of FIGS. 1A and 1B, the initial upward deflectionrequired for tip and tilt beamsteering as well as overall footprint forthe design may be tailored to any desired application through a numberof design and material modifications to include changing the beamlengths, changing selected materials of the bimorph beam structure,thickness of the beam material layers, the number of beams used fordeflection purposes, the residual stress and CTE properties of thematerials, the difference in the Young's modulus of the materials, thedeposition temperatures and composition of the material layers, and theoverlap of the various beams in the bimorph beams.

Some of the main differences in operational performance between theexemplary embodiments in FIGS. 1A and 1B include: 1) the center contactdesign exhibits a higher pull-in voltage, 2) the structural reliabilityof the center contact design is greater than the serpentine design, 3)the overall spring constant for the center contact design is muchgreater than the serpentine design, which may be advantageous in highvibration environments, and 4) the resonant frequency of the centercontact is much higher than the serpentine design. Thus, there are awide range of applications that could integrate either configuration tomeet a desired application.

The exemplary embodiment of the serpentine design in FIG. 1B is anenhanced version of another exemplary 7-beam serpentine design as shownin FIG. 2 in which all bimorph actuator beams were set to the samelength. The serpentine design in FIG. 1B utilizes available surface areaof the wafer much better, which in turn aids in increasing the upwarddeflection. Both serpentine designs in FIGS. 1B and 2 provide lowvoltage pull-in for large out-of-plane deflections while maintaining ahigh fill-factor once coupled with the micromirror assembly as shown inFIG. 3 are not possible in most MEMS actuators. A disadvantage to thedesign in FIG. 2 where all beams were the same length, is that once thepull-in voltage is reached, the entire actuator structure collapses ontothe electrode, mitigating any downward deflection control. The modifieddesign in FIG. 1B improves on several concepts. First, the initialupward deflection can be increased significantly within the samefootprint from the original design as the bimorph beams are optimallylengthened to enhance upward deflection as illustrated in FIGS. 4A and4B (deflecting approximately 60 μm and 145 μm respectively). And,second, the actuators spring constant is varied with respect to thebimorph beam lengths with the lowest spring constant coming from thelongest beam and steadily increases as the bimorph beams are shortenedapproaching the bonding platform. Thus, downward control of themicromirror tip, tilt, and piston deflection is possible simply throughvarying the bias voltages on the actuator electrodes.

FIGS. 5A and 5B illustrate the effect of changing the metal layer on theperformance of the center contact design of FIG. 1A with identicalphysical bimorph beam dimensions. The only difference is the metal layerchosen within the bimorph beam from aluminum to gold. The initial upwarddeflection for the aluminum beam shown in FIG. 5A is approximately 150μm while FIG. 5B shows the gold bimorph beam with only about 80 μm ofdeflection. For this minor material modification, the primary reason forthe deflection difference was the difference in CTE and the differencein Young's modulus of the materials.

FIG. 3. illustrates the center contact structural design concept with anSOI micromirror bonded onto the bonding pad of the actuator assembly.FIG. 3 shows the basic design concept 10 with the cantilever style beams12 attached to a bonding platform 14. This bonding platform 14 is usedto bond and support the micromirror pillar 16 and mirror plate 18 toenable the deflection and piston motion. In this design, the entirestructure used for actuation will be fabricated on a single wafer whilethe micromirror pillar and mirror will be fabricated from an SOI waferor some other surface micromachining technique. FIG. 6 illustrates themodeled upward deflection created by the inherent residual stress, CTEdifference, and the different Young's modulus in the cantilever likebeams to provide the initial, post-released peak upward displacement.All remaining deflections and piston motion will occur due tocontrolled, user-driven cantilever beam deflection. The overallintegration between the micromirror and the actuation technique may bethrough basic adhesion bonding to the actuation platform through the useof epoxy or metal-to-metal fusion bonding, though other bonding methodsmay also be used.

Embodiments of the invention are based on different actuator designconcepts, which are tailorable to meet a wide variety of applicationspecifications. These designs may be fabricated using a wide range ofmaterials to create the large out-of-plane upward deflections fromsurface micromachining principles to enable the large angle tip/tilt andpiston motion to properly steer an optical beam. From these designconcepts, large angle beamsteering can be performed while alsoexhibiting a high fill-factor for optical applications. Through somebasic design changes to the center contact design, this design may beused as either an electrostatic or an electrothermal design. The actualselection between the electrostatic or the electrothermal design isdependent on the application. An electrostatic design will requirehigher voltages to actuate the structure, but will benefit fromswitching speeds that will be much faster than an electrothermal design.The electrothermal design will require lower voltages and will exhibitlarger power consumption than the electrostatic design. However, theelectrothermal design will also enable larger forces to be generated andapplied to the structure for implementing the tip/tilt and pistonmotions.

A large out-of-plane deflection is the first stage in developing a largeout-of-plane beamsteering technique for a surface micromachined device.In general, the large out-of-plane deflections may be achieved bycapitalizing on the materials inherent residual stress and Young'smodulus as well as the difference in the materials coefficient ofthermal expansion (CTE) to form a traditional bimorph design. Inaddition, the material thickness, beam lengths, the number of beams, andthe deposition of the multi-layers which make up the beams allsignificantly contribute to the peak out-of-plane deflection. The upwarddeflections can be tailored to the application need as nearly allmetals, dielectrics, semiconductor, and polymer materials can be used tocreate these devices. The only caveat to this is during the fabricationprocesses, one needs to select materials that can withstand the variousetching and patterning processes. The illustrated embodiments of anactuation system are made up of four individually controlled bimorphactuators which enables system tip/tilt motion to angles of ±45 degreesas well as provide a piston motion if all four actuators are biasedsimultaneously. These are low voltage actuation systems which operate onelectrostatics (less than 100V) to pull down the actuators to create therequired forces to tip/tilt or piston drive the overall system.Electrothermal designs are again based on the traditional bimorphstructure but joule heating is the actuation mechanism at less than 10V. A preferred configuration for the overall system would be an array ofthese actuation structures which exhibit a high fill-factor greater than90% to mitigate signal loss and maximize beam reflection/detection.

As shown in FIGS. 1A and 1B above, to maximize the out-of-planedeflections, different material layers are used to create bimorph beamstructures of an actuation assembly. The actuator design conceptcapitalizes on the residual stress and the coefficient of thermalexpansion (CTE) differences between the layers. An exemplary layout 20is shown in FIG. 7A. In the exemplary embodiment illustrated in FIG. 7A,an electrode 22 is formed on a substrate 24. A sacrificial layer 26 isformed over the electrode. An insulator 28 is then formed over thesacrificial layer 26 and a portion of the substrate 24. Finally, a metal30 is formed over the insulator 28, where a portion 32 of the metal 30is under a tensile stress. When the sacrificial layer 26 is removed, theconfiguration is “released” and a portion 34 of the metal layer 30deforms due to the prestressed condition as shown in FIG. 7B. FIG. 7Cshows a scanning electron microscope (SEM) image of the design conceptas fabricated in the PolyMUMPs foundry process.

The electrostatic and electrothermal actuation systems may be madeutilizing surface micromaching in which thin material layers aredeposited and photolithography patterned on the surface of the wafer.These actuators may be developed in a wide variety of materials,deposition techniques, and fabrication facilities, even to includeavailable commercial foundries. This design concept can be used as a keycomponent in a wide variety of large angle beam steering approaches forplatforms and UAVs. The structures may also be used for imaging andscene generation.

There are a wide range of alternatives to these electrostatic andelectrothermal actuators. All one needs to verify is the residualstresses, Young's modulus, and the CTE of the selected materials meetthe application requirements. The greater the difference in CTE values,the greater the possible deflections. Generally, a conductive layer 30is required to create the lower electrode 22 and a second conductivelayer as part of the actuation platform to enable the electrostaticattraction for device operation, which is similar to the electrothermaldesign with the lower electrode 22 not being required. These metallayers could be gold, aluminum, chromium, titanium, platinum, copper,and nickel, among others, while the dielectric layers could be silicondioxide, silicon nitride, hafnium oxide, and aluminum oxide, amongothers. Various polymers could also be used to create these devicestructures but care would need to be taken to mitigate possible etchingduring the patterning and development of the structures. One of the keyconcepts in creating these structures is in the material selection suchthat there is a fairly large difference in CTE as well as the Young'smodulus of the material. A higher Young's Modulus will create a morerigid and stable structure but there are limits as bending must occur tocreate the tip and tilting of the platform from the actuators.

A MEMS commercial foundry may also be used in addition to in-housefabrication efforts to make these large out-of-plane structures. Asillustrated in the various figures, a designer has a wide range ofoptions to meet there desired application goals from the physical sizeof the device, to material selections, to residual stress levels withinthese layers. From these options, designers can create low angletip/tilt/piston driven devices to very large out-of-plane structureswhich enable large angle tip/tilt and piston motion.

The above illustrated embodiments of the invention are based on the useof the PolyMUMPs foundry fabrication as a baseline or foundation of theoverall system. From this foundry, large angle beamsteering whileexhibiting a high fill-factor is not possible. Thus, from the baselineprocess, several post-processing steps may be performed to enable thelarge out-of-plane upward deflections to permit large anglebeamsteering. Initial as fabricated structures from the foundrygenearlly provide a peak out-of-plane deflection of approximately 11 μmto 140 μm, depending upon the design. Performing post-processingdepositions of high temperature gold and a compressively stressedsilicon nitride layer on these same designs, the peak out-of-planedeflections increase from greater than 200 μm to over 1 mm. Thesepost-processing methods are viable for both electrostatic andelectrothermal designs.

The electrostatic and electrothermal actuation systems are constructedutilizing surface micromaching in which thin material layers aredeposited and photolithography patterned on the surface of the wafer.The actuators may be developed in the PolyMUMPs foundry process asoutlined below with additional post-processing steps to includehigh-temperature gold evaporation and PECVD silicon nitride layersdeposited prior to the final release. This design concept may be used asa key component in a wide variety of large angle beam steeringapproaches for platforms and UAVs. The structures can also be used forimaging and scene generation.

The foundation of the exemplary designs use the PolyMUMPs fabricationprocess which is outlined in Cowen et al., “PolyMUMPs™ Design Handbook,”Revision 13, which is incorporated by reference herein in its entirety,though other fabrication processes may also be used. FIG. 8A illustratesa cross sectional view of all deposition layers and the table in FIG. 8Boutlines each layer thickness and layer functionality. The surfacematerial layers may be deposited by low pressure chemical vapordeposition (LPCVD). The sacrificial oxide layers, which consist ofphosphosilicate glass (PSG) for this illustrated example, serve twopurposes. First, they define the gaps between structural layers, andsecond, they serve as a dopant source for the 1050 C high temperaturephosphorus diffusions, which assists in reducing the resistivity in thepolysilicon structural layers. All surface layers may be patterned usingstandard photolithography techniques and etched using Reactive IonEtching (RIE) or other etching methods. The final surface layer, a 0.5μm-thick gold metallization layer with a 100 nm chrome adhesion layer isdeposited and patterned using a standard lift-off technique. Lastly, arelease etch is performed to remove the sacrificial oxide layers freeingthe structural polysilicon layers (Poly1 and Poly2). The typical releaseetch is performed by immersing the die in room temperature hydrofluoric(49%) acid for 2-3 minutes, methanol rinses to stop the HF etch, andthen a supercritical carbon dioxide (CO₂) rapid dry to minimize stictionof the actuation assemblies. Note that for the electrothermal actuatorsembodiments designed in this process, only the Poly2 and gold layersneed be used to create a foundation for further device developmentthrough the addition of other material layers.

In order to implement the post-processing steps, which must be completedto enable the large out-of-plane deflections, a series of masks areneeded to define the construction of the additional material depositionpatterns for the beam structures. FIGS. 9A-9C illustrate thepost-processing fabrication process. The process begins with a baselinefoundry fabrication 60 from PolyMUMPs including a silicon dioxide layer62, polysilicon layer 64, and gold layer 66 on a 1 cm² silicon die 68 asshown in FIG. 9A. From this baseline, the PolyMUMPs gold layer 66 may beused or it may be etched off and redeposited with a high temperature (upto 300 C) gold evaporation layer. Following this gold deposition, a highcompressive stressed silicon nitride layer 70 is deposited using PlasmaEnhanced Chemical Vapor Deposition (PECVD) at a thickness of 1 μm andthen photo lithographically patterned using AZ5214 photoresist.Following the UV exposure and development, the silicon nitride layer 70is etched using reactive ion etching (RIE) and then the remainingphotoresist is removed using acetone. Following the deposition andpatterning of the silicon nitride layer, the three layer stackedmaterial beam structures which make up the actuation assembly iscompleted and shown in FIG. 9B. Finally, the sacrificial silicon dioxidelayer 62 may be removed using 49% hydrofluoric acid (see FIG. 9C) whichis then followed by a CO₂ critical point dry to fully release and drythe actuation assembly. While certain deposition and removal methodswere used with this illustrated embodiment, other deposition and removalmethods may also be used.

COMSOL® finite element modeling (FEM) software was used to model the preand post-processed foundry fabricated MEMS designs to determine theout-of-plane deflections. Based on the design constraints of the foundryprocess and an allotted design space criteria for a single element (1mm²), the PolyMUMPs foundry does not meet the required deflections asshown in the COMSOL® simulation shown in FIG. 10 of the structure. Asseen in FIG. 10, there is an overall peak deflection of zero microns.This is due to the full bimorph beams lacking a bending moment componentwhich can force the beam tips to deflect downward, creating an elongated‘S’ shaped final profile. In the current PolyMUMPs foundry fabrication,there are no additional material layers available which can be used tocreate this bending moment.

As illustrated in FIG. 10, the image illustrates the bimorph beamsupward bending which when coupled with a silicon nitride (SiN)deposition placed on top of the gold/polysilicon stacked beams, canindeed create the necessary bending moment to cause the actuationassembly to deflect upward as shown in FIGS. 11 and 12. Therefore,through the use of the PolyMUMPs structural layers as the foundation ofthe actuation assembly, post processing depositions of SiN (FIG. 11) andhigh temperature gold metal evaporation (up to 300 C) (FIG. 12) may beused to achieve the upward deflections needed for large angle beamsteering. FIG. 11 illustrates the as fabricated PolyMUMPs electrothermaldesign with a SiN layer deposited by plasma enhanced chemical vapordeposition (PECVD) at a stress level of −200 MPa at 300 C shows adeflection of ˜50 μm. In FIG. 12, the PolyMUMPs gold layer was removedand replaced with a gold layer deposited at 300 C. A −500 MPa PECVD SiNlayer was then deposited at 300 C. As shown in FIG. 12, over 250 μm canbe achieved using these post-processing techniques. The electrothermalPolyMUMPs design provided an out-of-plane deflection of 162±5 μm using acoupled SiN layer exhibiting a stress level of −930 MPa. All deflectionsare determined using white light interferometry (IFM).

The post-processing steps outlined above were repeated for anelectrostatically actuated design utilizing a beam structure in the formof a folded cantilever beam or serpentine layout. The baselineelectrostatic serpentine design fabricated in the PolyMUMPs fabricationprocess resulted in an out-of-plane deflection of ˜140 μm as shown inthe COMSOL® image in FIG. 13A. FIG. 13B illustrates a 5×4 array of thesestructures with a close up view shown on the right of a single actuationassembly. The out-of-plane deflection was measured to be ˜148 μm usingan IFM. This deflection does not meet the out-of-plane deflectionsrequired for large angle beam steering; thus, a PECVD SiN layer wasdeposited with the same compressive stress level of −930 MPa aspreviously stated. This resulted in an experimentally measuredout-of-plane deflection of over 1 mm. This is far too high so the PECVDdeposition parameters for this design will need to be adjusted to reducethe stress level in the nitride layer. The electrostatically actuatedcenter contact design has deflections greater than 430 um.

FIGS. 14A-14D provide additional exemplary L-Edit torsional springdesign attachments, which may be used, and which exhibit varying degreesof torsional and twisting stiffness. The platform/spring assembly shownin FIG. 14A provides a moderately high spring constant for both thepiston and tip/tilt motions, FIG. 14B provides a high spring constantwhich is ideal for rigidity and reliability of the platform assembly butalso nearly mitigates any piston or tip/tilt motions when a 2 μN forceis applied at Point A and B. FIG. 14C illustrates a lower springconstant which makes the platform motions resulting from forces appliedto Points A and B greater but still results in the opposite actuatormoving downward, reducing the peak tilt angle achievable. FIG. 14Dprovide the best spring constants for this project as it allows forsignificant deflection of the platform resulting from either a forceapplied to Point A or B. However, the spring constants are high enoughto maintain structural integrity following either actuation event.

The images shown in FIGS. 14A-14D show several exemplary completemicromirror designs illustrating the torsional spring/platform assemblydesign as shown in FIG. 14D with the platform assembly boxed in FIG.15A. FIG. 15A provides a top image of the complete unreleasedmicromirror actuation design with the actuation assembly and platformassembly integrated to form one element of a micromirror actuationarray. The structures shown in FIGS. 15A-15C were fabricated in thePolyMUMPs™ process such that the torsional spring attachment is ˜1.5 μmthick with a width of 8 μm. FIG. 15B illustrates a released micromirroractuation assembly which is deflected ˜273 μm out-of-plane with thespring attachment boxed and reimaged in FIG. 15C when the spring isfully deflected. As can be observed in FIG. 15C, the spring is flexibleenough to not fracture during full actuation conditions.

This realization of several exemplary torsional spring attachments werepresented, which is the critical linkage between the micromirroractuation assembly and the platform assembly. COMSOL® models were usedto assess the viability of the various torsional spring designs forrigidity and flexibility to perform piston motion as well as fortip/tilt motion. The fabricated structures were presented which clearlyshows the torsional spring does not fracture when in its fully extendedposition. As set forth above, choices of materials with these geometriesaffect performance and may be tuned to specific requirements andapplications.

While the present invention has been illustrated by a description of oneor more embodiments thereof and while these embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope of the general inventive concept.

What is claimed is:
 1. A MEMS device on a substrate, comprising: aplatform; an actuator assembly composed of a plurality of actuatorelements, the actuator assembly connected to the platform; whereinactuation of the plurality of the actuator elements in the actuatorassembly causes motion in the platform.
 2. The MEMS device of claim 1,wherein each actuator element of the plurality of actuator elements hasa first end and a second end, wherein the first end of a first actuatorelement of the plurality of actuator elements is connected to thesubstrate and the second end of a last actuator element of the pluralityof actuator elements is connected to the platform, and wherein the firstends of the remaining plurality of actuator elements are connected tothe second ends of other actuator elements between the first and lastactuator elements of the plurality of actuator elements to form a chain.3. The MEMS device of claim 1, wherein the plurality of actuatorelements in the actuator element chain is arranged in a serpentineconfiguration.
 4. The MEMS device of claim 1, wherein the plurality ofactuator elements in the actuator element chain is arranged in a centercontact configuration.
 5. The MEMS device of claim 1, furthercomprising: a micromirror bonded to the platform.